A device for measurement of basic sediment and water (BS&W) in a predominantly non-aqueous stream (e.g. pipeline crude oil) is disclosed in U.S. Pat. No. 4,184,952. This device is an improvement on capacitance type instruments of the art which are dependent upon the extent to which the intrinsic dielectric constant of the subject fluid varies with time. The gravity and physical composition of crude oil are two factors which determine its intrinsic dielectric constant. If one or both of these properties should vary, instruments measure the accompanying change in the dielectric constant as percent BS&W. This yields an inaccurate measurement of BS&W because instruments must be initially set to read zero BS&W as the intrinsic dielectric constant of the fluid. The capacitance type instruments of the prior art have no means for automatically correcting the zero BS&W setting to compensate for periodic variations in the oil properties mentioned. By comparison, the device of U.S. Pat. No. 4,184,952 provides for automatic compensation of BS&W measurements by producing a clean, dry sample of the line fluid for measurements of its intrinsic dielectric constant. In this way, the true BS&W content of the fluid is measured by finding the difference between the dielectric constants of the wet and dry streams.
Even though the improvement over the art represented by the invention of U.S. Pat. No. 4,184,952 is substantial, it now has been discovered that other improvements can be made which even further improve the efficiency and accuracy of this invention. Thus, variance in flow between between the cells utilized for measuring capacitance in the wet and dry streams of the BS&W recorder can cause loss of accuracy of readings. For example, with the device disclosed in U.S. Pat. No. 4,184,952, the flow through a cell utilized to measure capacitance of a wet stream would be about 5 to 100 times that through a cell utilized to measure capacitance of a dry stream. This cannot be avoided because isokinetic sampling and sampling tube diameter require relatively high wet stream flow, while dry stream output from the centrifuge of the apparatus of U.S. Pat. No. 4,184,952 is relatively low. The low velocity in the dry cell allows (1) accumulation of residue and causes (2) a time delay of several minutes in the dry cell perception of the arrival of a new batch of oil. Since the time delay is fairly constant, it can be calculated out by computer or microprocessor, but elimination of time delay and increasing velocity in the dry cell are both desirable goals.
It would initially appear that an improvement to the BS&W instrument would be to reduce the size of the dry cell so that the velocity of the oil through it would approximately equal flow through the wet cell. However, having cells of two different sizes would introduce complex calibration problems. Thus, it is desirable to have both the cells identical.
As noted, one problem caused by low velocity in the dry cell is the accumulation of residue such as wax. Such residue may result not only from low velocity but also from temperature variations as mentioned in applicants' co-pending application Ser. No. 291597 filed Aug. 10, 1981. Thus, the cell consists of two coaxial tubes electrically insulated from each other wherein the annular space between the tubes constitutes an electrical capacitor whose value depends upon the material in the annular space. Residue usually tends to cause an error in the direction of wetness. Thus, the dry cell indicates less dry than it should compared to the wet cell where flow is more rapid and residue does not tend to accumulate.
Accordingly, the present invention provides solutions to overcoming the above noted problem wherein relatively slow flow in the wet cell allows accumulation of residue.